1. Field of the Invention
The present invention relates to a reception apparatus.
2. Description of the Related Art
A DS-CDMA (Direct Sequence-Code Division Multiple Access) radio access scheme is known, in which a plurality of users perform communications by using the same frequency band. In the DS-CDMA radio access scheme, users are identified by using spreading codes.
In mobile communication, multiple propagation occurs due to reflection, scattering, and refraction caused by surrounding structures, trees, and the like. Incoming waves reach a reception point through propagation paths having different lengths. The amplitude and phase of each incoming wave vary depending on the position. A variation distribution can be approximated by a Rayleigh distribution when the signal arrives by way of an indirect path.
In the DS-CDMA radio access scheme, since information data is band-spread by using a high-rate spreading code, the data can be separated into paths having a propagation delay difference larger than the period of the spreading code. By adding the phases of a plurality of separated multipath signals, a diversity effect can be obtained to attain an improvement in reception characteristics.
Since the position of a mobile station varies relative to a base station, the delay profile varies accordingly. In mobile communication, therefore, a receiver needs to have the function of absorbing such variations in the respective paths and combining a plurality of multipath signals in phase. This variation speed increases with an increase in the speed of the mobile station. To perform communication under a high-speed moving state, high-precision channel estimation that can follow fading variations is required. In a DS-CDMA radio access scheme having a frame format in which pilot symbols are inserted between information symbols in predetermined cycles, several channel estimation schemes for absorbing fading variations have been proposed.
FIG. 7 shows an example of the frame format used in the DS-CDMA radio access scheme. A basic concept of a channel estimation method will be described with reference to FIG. 7.
Referring to FIG. 7, “time slot” represents cycles in which pilot symbols are inserted between data symbols. Pilot symbols are inserted between data symbols in time slot cycles Tp. One cycle Tp (one frame) consists of Np pilot symbols and Nd data symbols. Channel estimation values at pilot symbol positions are obtained by using the pilot symbols in each time slot. By combining these channel estimation values using some method, a channel estimation value at each data symbol point is obtained.
According to Sanpei, “Rayleigh Fading Compensation Method for 16QAM MODEM in Digital Land Mobile Radio Systems ”, IEICE B-II, Vol.J72-B-II, No. 1, pp. 7-15 (1989-01) (reference 1), interpolation is applied to a channel estimation value obtained by a pilot symbol and a reception symbol. In addition, according to Honda and K. Jamal, “Channel Estimation based on Time-Multiplexed Pilot Symbols”, IEICE RCS96-70 (1996-08) (reference 2), a channel estimation value corresponding to a data symbol is obtained by averaging processing.
FIG. 8 is a graph showing a comparison between characteristics obtained by using a primary interpolation method and averaging method as channel estimation methods. Referring to FIG. 8, the abscissa represents the maximum Doppler frequency normalized with the pilot symbol insertion cycle; and the ordinate, the average channel estimation error in decibels. As is obvious from FIG. 8, the channel estimation errors are small when the averaging method is used as a channel estimation method in an area where fading variations are small (low-speed movement environment) and the interpolation method is used as a channel estimation method in an area where fading variations are large (high-speed movement environment). As a consequence, good BER (Bit Error Rate)/FER (Frame Error Rate) characteristics can be obtained.
The above methods are typical channel estimation schemes. FIG. 9 shows the arrangement of conventional RAKE receiver using these schemes. Referring to FIG. 9, after a reception signal is de-spread by a matched filter 700, a pilot symbol is detected. A channel estimation device 701 then performs channel estimation according to reference 1 or 2. The conjugate complex number of a channel estimation value calculated in each path, and this value is multiplied by the output from the matched filter 700 by a multiplier 702, thereby performing delay compensation. The resultant data are then combined at a maximum ratio by a RAKE combiner 703. The combined signal is subjected to symbol decision in a decision device 704 and converted into a bit stream. The bit stream data obtained in this manner is de-interleaved by a de-interleaver 705. The resultant data is then Viterbi-decoded by a Viterbi decoder 706, thus reconstructing transmission data.
In a low-speed movement environment, channel estimation based on averaging is better in characteristics than channel estimation based on primary interpolation. In contrast to this, in high-speed movement environment, primary interpolation exhibits better characteristics. For this reason, good characteristics cannot be obtained throughout the entire moving speed range.
If, however, a channel estimation method is fixed in a use environment as in the prior art, a deterioration in communication quality occurs in either a low-speed movement environment or a high-speed movement environment when the use environment deviates from the expected environment. It is therefore difficult to realize high-quality communications seamlessly from indoor semi-fixed communications to high-speed mobile communications.